Friction and Wear in Threaded Surfaces of Rotary Drill Collars

Two surfaces, under high normal stress, in sliding contact provide the basis for friction and wear studies within rotary drill collars. Flat and ring specimens, considering three (3) different contact areas, were rotated to determine the effect of surface finish, coatings, lubricants and normal stress on friction and wear. The 4145 steel specimens were heat-treated to a yield strength of 124,000 lb/in² (855 MPa) and a R_c hardness of 28. The torque required to rotate the ring specimen was measured as a function of the rotation angle. The friction coefficient was calculated. Seizure and galling were common for metal-to-metal contact. Rust and phosphate coatings break down under the high normal stress. Metal-filled lubricants produce static coefficients of friction (μ_s) between 0.03 and 0.25 and dynamic coefficients between 0.04 and 0.26. Seizure and galling were not observed.